1. Field of the Invention
The present invention refers to an electric circuit and an oscillator comprising said electric circuit.
2. Description of the Related Art
Circuit structures are generally known that supply triangular or square waveform signals. Said circuits are frequency oscillators and are widely used in digital and analog circuitries. The main characteristics of a good oscillator are to operate at high frequency and to have an oscillation frequency that is independent from the supply voltage, the variations of the process and the temperature and to have a duty-cycle defined by a constant ratio. Many oscillators however present inconveniences linked to the dependence of the oscillation frequency and of the duty-cycle on the values of the threshold voltages of the inverters.
A low supply voltage oscillator is described in EP 1049256. Said oscillator has a symmetrical structure and comprises two circuit stages having a specular circuit structure and placed between a supply voltage and ground. The oscillator also comprises two capacitors placed between the supply voltage and ground and said stages serve to charge and discharge said capacitors in alternated mode. The stages are connected together by means of a memory element formed by a bistable device or by a flip-flop. Each of the two circuit stages comprises a constant current generator suitable for charging the capacitor, a first MOS transistor having the gate terminal connected to the common terminal of the capacitor and of the current generator and a second MOS transistor placed in parallel to the capacitor. The capacitor is charged until the voltage at its terminals reaches the threshold voltage of the first MOS transistor; the discharge of the capacitor comes about by turning on the second MOS transistor. Thus an inconvenience of said oscillator lies in the fact that the charging time and therefore the oscillation frequency depend on the threshold voltage of the MOS transistor.